Dispersal ability of seed morphs predicts germination sensitivity to competition cues in a heterocarpic plant

Seed germination is often regulated by cues that indicate future competitive conditions, and such cues have been shown to either accelerate or suspend germination across species. However, much less is known about how these responses might vary within species, particularly in plants that produce seeds with different dispersal abilities. In seed-heteromorphic species, dispersing and non-dispersing seed morphs may differ in their likelihood of encountering kin competition, potentially selecting for contrasting germination strategies.

We studied the effect of light competition cues on the germination percentage and speed of dispersing and non-dispersing achenes (single-seeded fruits) of the heterocarpic plant Heterotheca subaxillaris. In a greenhouse experiment, we experimentally exposed both achene morphs to one of three light environments: full light, neutral shade (reduced irradiance), and simulated vegetative shade (reduced irradiance and red:far-red light ratio).

Dispersing, pappus-bearing achene morphs of H. subaxillaris exhibited consistently high germination percentage and speed, which were unaffected by the light treatments. In contrast, germination percentage of the non-dispersing morphs were strongly reduced under simulated vegetative shade and moderately reduced under neutral shade, compared to full light.

The results of this study are consistent with the hypothesis that non-dispersing seed morphs may reduce germination under cues associated with nearby vegetation, potentially contributing to temporal avoidance of competition near the maternal plant. These findings demonstrate that seed heteromorphism is associated not only with variations in dispersal and dormancy, but also with differential plasticity in germination responses to environmental cues.